Method for improving the control response of an antilock control system during braking operations in the existence of a high coefficient of friction

ABSTRACT

In a method for improving the control response of an anti-lock control system (ABS) during braking operations at a high coefficient of friction, the brake pressure control thresholds, in particular the intervention or application thresholds of the anti-lock control system (ABS) are adapted dynamically to the adherence abilities of the respective vehicle wheel or the tire.

BACKGROUND OF THE INVENTION

Anti-lock control operations take place in all ranges of coefficients offriction, beginning with a very low coefficient of friction (μ<0.1) onice until high coefficients of friction (μ=1) on dry asphalt.

To reach a shortest possible stopping distance, it is especiallyimportant that when braking on roadways with a high coefficient offriction the anti-lock control systems (ABS control systems) will nottake up their control activity before the maximum of the so-calledμ-slip curve is reached. Otherwise, the ABS system would not utilize thepotential of the tire. It would virtually impede the tire in developingthe μ-slip curve, and the adherence abilities of the tire would not beutilized in full extent.

To prevent this occurrence, the control thresholds for the ABS brakingoperation or the control application thresholds must be rated accordingto the high coefficient of friction conditions.

Recently, the trade press has assessed the braking performance or thestopping distance on dry asphalt at an increasing rate as the decisivecriterion for the quality of the tested ABS system. Therefore, themajority of car makers attribute great importance to these tests.

In some of these tests, the vehicles exhibit very high decelerationvalues. The same vehicles, as soon as they are equipped with differenttires, exhibit a distinctly differing braking performance in comparabletests.

Of course, the difference in the performance can be caused because theconditions of measurements differ from each other, e.g. due todifferences in the coefficient of friction of the roadway.

However, the specific tests show that the tires partly differ distinctlyin the amount of the coefficient of friction, that means the magnitudeof the transmittable μ of the μ-slip curve. This fact explains thesignificant differences in the braking performance of different tiremakes and types of tires under otherwise equal conditions (i.e.identical vehicle, on the same test track, identical climaticconditions, etc.).

The essential thing is to treat different tires in such a fashion thatthe potential of each individual tire is utilized in the best possiblemanner. This implies that, on the one hand, the tires with lowertransmittable adherence values (μ) are not overbraked (e.g. areexcessively subjected to brake slip due to decelerated use of the ABScontrol) and that, on the other hand, the tires with highertransmittable μ should not be underbraked (i.e. are not subjected tobrake slip at all due to a premature use of the ABS control). Theobjective rather is to design the ABS control in such a fashion thateach tire is controlled according to its optimum.

The anti-lock control systems (ABS systems) used in series do notcomprise longitudinal acceleration sensors apart from a few exceptionsin the field of all-wheel driven vehicles. Therefore, determining amaximum deceleration responsive to the roadway is only possible based onthe wheel speed signals. As long as the wheels are not yet braked, it isno particular problem to determine the vehicle deceleration. Things aredifferent, however, in the event of full braking because the wheels arealways afflicted by slip. In this case, the vehicle speed and thevehicle deceleration a_(veh)=Δv_(veh)/Δt can be determined only inapproximation with the use of the prior-art methods by determining andlogically combining the wheel rotational behavior of the individualwheels and selecting defined control phases.

In anti-lock control systems, the deceleration a=Δv/Δt, determinedeither according to the prior-art method or measured by means of alongitudinal acceleration sensor, represents the input value in aprogression term which is taken into account for calculating the controlthresholds and control application thresholds of the anti-lock control(ABS control). The deceleration and the so-called negative feedbackdefine the application of the ABS control. The negative feedbackrepresents the wheel deceleration value at which the stable branch ofthe μ-slip curve has not yet been left, i.e. wheel slip ‘does not yetshow’, because the maximum transmittable adherence or coefficient offriction (μ-value) has not yet been reached or, specifically, where theABS control has not yet commenced if it is designed properly.

The negative feedback is deducted from the currently prevailing wheeldeceleration a_(wheel) for calculating the control applicationthresholds. Only when the wheel deceleration exceeds the negativefeedback value will this be identified as a locking tendency, and thediscrepancy from the negative feedback is detected, integrated andevaluated. The integral represents an essential criterion for thedetection of an ABS situation, i.e. an ABS control operation or alocking tendency.

If the value of the negative feedback or of the control threshold is toolow, a tire with a relatively high transmittable adherence value (μ) issubjected to the ABS control prematurely, i.e. still in the stable rangeof the μ-slip curve, and, thus, is virtually hindered to utilize theinstantaneously existing tire/road adherence value. The stoppingdistance becomes longer than would be necessary in view of roadconditions and the adherence ability of the tire.

If, however, the value of the negative feedback or of the controlthreshold is too high, a tire with a relatively low transmittableadherence value (μ) is subjected to ABS control too late, i.e. only farin the unstable range of the μ-slip curve, and, thus, is virtuallyforced into deep slip up to a wheel lock condition. The result would bean ‘inhomogeneous’ ABS control with excessive pressure modulation. Thiswould cause major losses in comfort and a significant impairment of thebraking performance.

Therefore, an object of the invention is to develop a method permittinga still better adaptation of an ABS control system to the differentadherence values dependent on the wheel or on the type of tire.

SUMMARY OF THE INVENTION

It has been found out that this object can be achieved by a methodinvolving that during braking operations at a high coefficient offriction, the brake pressure control thresholds, in particular theintervention or application thresholds of the anti-lock control system(ABS) are adapted dynamically to the adherence abilities of therespective wheel, in particular the tire.

The above-described objective of the individual adaptation of the ABScontrol system to the tires can principally be achieved in two ways,i.e. by raising the control thresholds and/or by increasing the negativefeedback in dependence on the respective conditions.

According to a particularly favorable embodiment of the invention, thecontrol thresholds, on which the commencement of anti-lock controldepends, are raised progressively by increasing the negative feedback(F) according to the relationF=F ₀ +k _(g0)*(a−a ₀)+k _(gl)*(a−a ₁) . . . +k _(gn)*(a−a _(n))  (1),where “F₀=Feedback₀” refers to the minimum negative feedback value,where “a₀” represents the input value at which the progressioncommences, where the quantities “a_(1 . . . n)” are vehicle decelerationvalues at which the progression is switched over, and where “a” is thecurrent vehicle deceleration and “k_(g0 . . . gn)” are valuationfactors.

In another embodiment of the invention, the control thresholds (Th), onwhich the commencement of anti-lock control depends, are raisedprogressively according to the relationTh=Th ₀ +k _(ao)*(a−a ₀)+k _(a1)*(a−a ₁) . . . +k _(an)*(a−a _(n))  (2).

In this case, “Th”₀ is the minimum threshold value, “a₀” is the inputvalue of the vehicle deceleration at which the progression commences;the quantities “a_(1 . . . n)” are vehicle deceleration values at whichthe progression is switched over; “a” is the current vehicledeceleration and “k_(a0 . . . an)” are valuation factors.

The increase must be progressively dependent on the vehicle decelerationin both cases. The term ‘control thresholds’ refers to accelerationthresholds or thresholds derived therefrom, such as diverse derivativesor integrals as well as slip thresholds and reference speed.

In both cases, i.e. adaptation of the control thresholds and thenegative feedback, the application thresholds are influenced directly orindirectly by way of the measured or calculated vehicle deceleration.

The progression commences at the vehicle deceleration that can bereached by the ‘weakest’ tires. The vehicle deceleration of roughly 1 to1.1 g (‘g’ implies the acceleration due to gravity constant) representsa value relevant in practice. Of course, defined fixed values must beadded during uphill or downhill driving.

All tires reaching this input value without the ABS control setting inwill naturally develop a higher vehicle deceleration and automaticallyincrease the control thresholds or the negative feedback. This way thetires with a higher coefficient of friction are automatically controlledwith a higher threshold or negative feedback.

The attached illustrations and diagrams serve for the more detailedexplanation of the mode of operation of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings,

FIG. 1 shows essential components of an ABS control system in the formof function blocks.

FIGS. 2, 3 show diagrams for illustrating the course of the wheel speedand vehicle speed and of the wheel acceleration in the initial phase ofan ABS control operation.

FIGS. 4, 5 show diagrams for illustrating the mode of operation whenapplying a progressive negative feedback.

DETAILED DESCRIPTION OF THE DRAWINGS

According to the basic principle of an ABS control system as representedin FIG. 1, the input signals of the control system are procured by meansof wheel sensors S1 to S4 representative of the rotational behavior(speed v_(wheel) and acceleration dv/dt) of the individual vehiclewheels wheel 1 to wheel 4. In addition, an acceleration sensor L1 can beinstalled for determining the longitudinal acceleration of the vehicle.

Such circuit variants are known in the art. A vehicle (reference) speedis determined from these measured variables in a circuit B1 illustratedas a block, the said speed's variation finally representing the vehicleacceleration or vehicle deceleration a_(veh), respectively.

B2 symbolizes the negative feedback and/or the influencing of thresholdsaccording to the invention. In B3 the above-mentioned data is processedin a conventional manner in order to obtain control signals foractuators A, especially for brake pressure or brake force actuators forcontrolling the brake pressure or the brake force at the individualvehicle wheels.

FIGS. 2 and 3 depict the vehicle speed v_(veh) and the speed v_(wheel)of the controlled wheel under review and the variation or timederivative of this wheel speed a_(wheel). The values for the stepwiseadaptation of the control thresholds of the invention as explained inthe following are also plotted in FIG. 3.

The thresholds Th are increased according to the following equation (1).It applies for the progressive development of threshold values:

-   Th₀=base threshold    Th ₁ =Th ₀ +k _(ao)*(a−a ₀)    Th _(n) =Th ₀ +Th ₁ . . . +k _(an)*(a−a _(n))    Th=Th _(n) =Th ₀ +k _(ao)*(a−a ₀)+k _(al)*(a−a ₁) . . . +k    _(an)*(a−a _(n))

“Th₀” is the minimum threshold value. The value “a₀” represents theinput value, where the progression starts, the quantities a_(1 . . . n)”are vehicle deceleration values at which the progression is switchedover, “a”, being identical with a_(cc) in FIG. 3, is the current vehicledeceleration and “k_(a0 . . . an)” are valuation factors. The currentvehicle deceleration must exceed the respective switch-over value inorder that the individual parts of this polynomial are taken intoaccount.

The following applies for the increase of the negative feedback. Thenegative feedback is increased according to the equation (2):

-   F₀=base value of the negative feedback    F ₁ =F ₀ +k _(g0)*(a−a ₀)    F=F _(n) =F ₀ +F ₁ . . . +k _(gn)*(a−a _(n))  (2)

“F” is referred to as progressive feedback. “F₀=Feedback₀” is theminimum negative feedback value. The value “a₀” represents the inputvalue, at which the progression commences, the quantities“a_(1 . . . n)” are vehicle deceleration values, at which theprogression is switched over, “a” is the current vehicle decelerationand “k_(g0 . . . gn)” are valuation factors. The individual parts ofthis polynomial will apply as soon as the current vehicle decelerationexceeds the respective switch-over value.

The adaptation or increase of the negative feedback in some cases is thepreferred simpler variant in comparison to the increase of theacceleration thresholds because influencing by way of the vehicledeceleration can take place at a central location in the control system.

FIG. 4 serves to illustrate the function and the mode of operation of aprogressive negative feedback. The dotted straight lines in FIG. 4 applywhen the individual terms F₀, F₁, F_(n) of the above-mentioned negativefeedback function F are included.

In FIG. 5 the wheel acceleration integrals DVN are illustrated in thecurve course or control case according to FIG. 4. For example, a valueof −4 km/h could be provided for the base threshold Th₀.

When taking into account only the negative feedback value F_(o), theshaded surface AF₀ bound by the wheel speed V_(wheel) and the straightline F_(o) symbolizes the wheel acceleration integral DVN that isdecisive for determining the ABS application threshold (FIG. 5). Thesurface AF₁, AF_(n) becomes smaller when the further terms of thenegative feedback equation apply.

In both cases, that means both in the progressive increase of thecontrol thresholds and in the progressive increase of the negativefeedback, the progression is indicated as a sum of several linearfunctions. It is, of course, possible to choose any other form ofmathematical progression; the simple form has proven fully appropriatein practice.

1. Method for improving the control response of an anti-lock controlsystem (ABS) during braking operations at a high coefficient offriction, characterized in that brake pressure control thresholds, inparticular the intervention or application thresholds of the anti-lockcontrol system (ABS) are adapted dynamically to the adherence abilitiesof the respective wheel.
 2. Method as claimed in claim 1, characterizedin that the control thresholds, on which the commencement of anti-lockcontrol depends, are raised progressively by increasing the negativefeedback (F) according to the relationF=F ₀ +k _(g0)*(a−a ₀)+k _(g1)*(a−a ₁) . . . +k _(gn)*(a−a _(n))  (1), where “F₀=Feedback₀” refers to the minimum negative feedback value,where “a₀” represents the input value at which the progressioncommences, where the quantities “a_(1 . . . n)” are vehicle decelerationvalues at which the progression is switched over, and where “a” is thecurrent vehicle deceleration and “k_(g0 . . . gn)” are valuationfactors.
 3. Method as claimed in claim 1 or 2, characterized in that thecontrol thresholds (Th), on which the commencement of anti-lock controldepends, are raised progressively according to the relationTh=Th ₀ +k _(ao)*(a−a ₀)+k _(al)*(a−a ₁) . . . +k _(an)*(a−a _(n))  (2), where “Th”₀ is the minimum threshold value, “a₀” is the input value ofthe vehicle deceleration at which the progression commences, where thequantities “a_(1 . . . n)” are vehicle deceleration values at which theprogression is switched over, and where “a” is the current vehicledeceleration and “k_(a0 . . . an)” are valuation factors.